Cardiac arrest is a significant public health problem cutting across age, race, and gender. A positive impact on cardiac arrest survival has been demonstrated with the substantial reduction in time to defibrillation provided by the widespread deployment of automated external defibrillators (AEDs). Examples of AEDs are described in U.S. Pat. Nos. 5,607,454, 5,700,281 and 6,577,102.
Optimal resuscitation therapy for out of hospital (OOH) cardiac arrest is the subject of substantial ongoing research. Research has been clear in demonstrating that the timing of resuscitation is of critical importance. For example, there is less than a 10% chance of recovery after just ten minutes after the onset of ventricular fibrillation (VF). This knowledge led to the recent widespread deployment of AEDs, primarily in public areas with a high population concentration such as airports and shopping malls. A positive impact on cardiac arrest survival has been demonstrated due to the substantial reduction in time to defibrillation as a result of more available access to AEDs.
Recent studies, however, have identified the importance of performing CPR-type chest compressions before defibrillation and minimizing the time to defibrillation shock following the cessation of the CPR chest compressions in facilitating effective recovery from VF episodes of especially long duration. It is generally believed that perfusion of the myocardium achieved during CPR preconditions the heart for the defibrillating shock. Despite the importance of CPR, implementation of CPR in the field is hampered by many problems including the dependence on rescuer technique, which is known to be variable even with trained professionals, fatigue over time, and attitude of the rescuer. Even in situations where an AED provides voice prompts instructing rescuers to administer CPR, rescuers perform CPR less than half the time in an actual rescue situation. A lack of understanding and fear of accidentally being subjected to energy from the defibrillation shock may make it difficult to induce non-professional rescuers using an AED to perform CPR up until the moment of defibrillation.
Conventional AEDs perform a cardiac rhythm analysis to determine if a patient has a condition that is treatable by a defibrillation shock. The cardiac rhythm analysis is performed just prior to shock delivery. Because CPR administered by a rescuer can interfere with a proper cardiac rhythm analysis, conventional AEDs provide a voice command prompt to stop performing CPR and not touch the patient during cardiac rhythm analysis. Some AEDs also utilize a time delay prior to delivering the defibrillating shock to reduce the risk of the non-professional rescuer being shocked. Studies have demonstrated that return of spontaneous circulation (ROSC) in the patient is most successful when defibrillation is administered during CPR. Furthermore, delays between CPR and defibrillation as short as 20 seconds have been shown to significantly reduce ROSC probabilities. Therefore, a need exists for a solution to minimize or eliminate such delays during rescue events utilizing AEDs.
The standard for electrical cardio-therapy administered from the exterior of the patient during ventricular fibrillation has been high-voltage, high-energy defibrillation signals. U.S. Pat. No. 6,298,267 describes the use of high-energy signals for treating ventricular fibrillation, and for restoring an effective cardiac output to relieve electromechanical dissociation or pulseless electrical activity conditions. For treating arrhythmia conditions, cardiac pacing therapy utilizing lower-voltage, lower-energy pacing signals is known. Externally applied pacing signaling functionality has been combined with defibrillation-type functionality in a single external device, as described in PCT Application, Publication No. WO 99/03534.
Cardiac electrotherapy signaling having an amplitude that is greater than that of pacing-type signaling, but less than the amplitude and energy level associated with defibrillation-type signaling, is known in the art as medium voltage therapy (MVT). For example, U.S. Pat. No. 5,314,448 describes delivering low-energy pre-treatment pulses followed by high-energy defibrillation pulses, utilizing a common set of electrodes for both types of signals. According to one therapeutic mechanism of this pre-treatment, the MVT pulses re-organize the electrical activity within the cardiac cells of the patient to facilitate a greater probability of successful defibrillation with a follow-on defibrillation pulse. U.S. Pat. No. 6,760,621 describes the use of MVT as pretreatment to defibrillation that is directed to reducing the likelihood of pulseless electrical activity and electromechanical dissociation conditions as a result of the defibrillation treatment. The mechanism by which these results are achieved by MVT has been described as a form of sympathetic stimulation of the heart. These approaches are directed to influencing the electrochemical dynamics or responsiveness of the heart tissues.
MVT has also been recognized as a way of forcing some amount of cardiac output by electrically stimulating the heart directly with signals that cause the heart and skeletal muscles to expand and contract in a controlled manner. See U.S. Pat. Nos. 5,735,876, 5,782,883 and 5,871,510. These patents describe implantable devices having combined defibrillation, and MVT capability for forcing cardiac output. U.S. Pat. No. 6,314,319 describes internal and external systems and associated methods of utilizing MVT to achieve a hemodynamic effect in the heart as part of an implantable cardioverter defibrillator (ICD) for purposes of achieving a smaller prophylactic device. The approach described in the '319 patent uses the MVT therapy to provide a smaller and less expensive implantable device that can maintain some cardiac output without necessarily providing defibrillation therapy.
One drawback associated with the existing MVT approaches for forcing cardiac output is they are not well-suited for out-of-hospital or external treatments. In the case of the MVT therapy described in the '319 patent, an implantable device must be implanted in each patient. The '319 patent expressly teaches that MVT therapy is not relevant to external devices, because such external devices are too slow in their arrival and use with a patient.
While developments in defibrillator technology, both automatic external defibrillators (AEDs) and implantable cardioverter defibrillators (ICDs) have made great strides in aiding the electrical cardiac resuscitation of individuals experiencing cardiac arrest, a need exists for a solution that can effectively induce respiration in a patient while electrically inducing coronary output in out-of-hospital rescue situations.